Japanese patent application laid-open No. 5-129374 discloses a method of making an electrode bump such that a microsphere is mounted on a bump forming member. After mounting the microsphere, the microsphere is fused. In this method, microspheres are adsorbed into holes that are provided in the same arrangement as that of the bump forming member, and then they are transferred onto the semiconductor chip or circuit board. Examples of bump forming members are semiconductor chips (semiconductor devices) and circuit boards and solder balls are examples of a microspheres.
An adsorption head is provided that has adsorption holes for microspheres in the same arrangement as that of the bump forming member. The adsorption head is moved to the bump forming member while holding the microspheres in the adsorption holes. Then, by releasing the microspheres, the microspheres are transferred to the bump forming member.
In this case, it is necessary to adsorb the microspheres onto the adsorption head in the correct proportion. However, since the microspheres are laid at random, it is difficult to adsorb them at predetermined positions when a vacuum is used to adsorb the microspheres. Therefore, a microsphere arrangement pallet is prepared such that microspheres are disposed in advance in the same arrangement as that of electrode bumps. By vacuuming the microspheres on the pallet, they can be adsorbed in correct proportion to the adsorption head.
However, it is difficult to stably arrange microspheres on the microsphere arrangement pallet in atmosphere. Due to static electricity or moisture, the microspheres may be adhered to each other or be adsorbed upon the surface of arrangement pallet.
Japanese patent application laid-open No. 11-8272 discloses a method that an arrangement pallet is soaked in conductive liquid and then microspheres are dropped on the arrangement pallet, secured in respective arrangement holes, thereby removing the influence of static electricity or moisture.
In the above in-solution arrangement method, where micro-metal balls (microspheres) are arranged in conductive liquid, a stable arrangement operation can be conducted because of removing the influence of static electricity or moisture. However, since it uses ethanol, which is highly volatile, it is necessary to supplement evaporated portion so as to stabilize the operation. Thus, a large amount of ethanol is needed. Further, in transferring the arrangement pallet to the next step, it is difficult to remove the pallet from the conductive liquid. Thus, it is difficult to automate the taking-out step.
Japanese patent application laid-open No. 2001-210942 discloses a method where another closed vessel is prepared, other than a closed vessel that used when the arrangement operation is conducted, while soaking the arrangement pallet. The vessels are then connected through a flexible tube. The conductive liquid and micro-metal balls are transferred between the vessels using the gravity difference of the vessels.
In this method, the evaporation of conductive liquid can be prevented by using the closed vessel, and the usability of employed material can be improved by using repeatedly the conductive liquid and micro-metal balls. Furthermore, the operation can be facilitated such that, after completing the arrangement operation on the arrangement pallet, the conductive liquid and micro-metal balls in the arrangement pallet soaking vessel are evacuated while being transferred to the other vessel, and then the arrangement pallet is taken out from the one closed vessel.
Subsequently, the microspheres are secured in the arrangement holes and then adsorbed by the adsorption head of a vacuum apparatus. The adsorption head has a plane with air holes provided therein corresponding to the arrangement holes. The adsorption is conducted such that the adsorption head is in contact with the arrangement-holes forming surface of arrangement pallet, and then the microspheres are adsorbed to the air holes under vacuum. After adsorption, the microspheres on the adsorption head are aligned with the pad position of semiconductor wafer, and then by cutting off the vacuum the microspheres are dropped on the pads to mount them there.
However, in the conventional method of mounting the microspheres on the bump forming member of semiconductor device by using the arrangement pallet, there are some problems described below.
Since the process needs to be conducted such that the microspheres are accommodated in the holes (arrangement holes) in the arrangement pallet and then transferred onto the pads of semiconductor device, the number of steps in the bump electrode forming process must be increased. The manufacturing cost will also be increased. Further, the entire composition of bump electrode forming process is quite complicated.
Further, in recovering the conductive liquid and excess microspheres to reuse them, the microsphere may be deformed, crushed or defaced due to hitting the corner of vessel or being trapped in the gaps of vessel. In this case, the microsphere may not be accommodated in the arrangement hole or may not be adsorbed by the adsorption head even when accommodated. Thus, the microsphere cannot be transferred to the semiconductor device. Furthermore, although the conductive liquid and microspheres can be recovered by tilting or reversing the vessel, a small amount of them may be left in the vessel. Thus, they are wasted to some degree.
Further, in adsorbing the microsphere into the adsorption head, the surface of adsorption head needs to be in close contact with the arrangement holes forming surface of the arrangement pallet. Therefore, high processing accuracy is needed to flatten both surfaces. Further, if the step of adsorbing the microsphere to the adsorption head is conducted in atmosphere, the neighboring microspheres attract each other, thereby failing to be adsorbed. As a result, the manufacturing cost in the bump electrode forming process is increased.
Still further, in order to enhance the operating efficiency, a process of accommodating microspheres in another arrangement pallet needs to be conducted during the adsorption. Therefore, multiple arrangement pallets are needed and the operating cost is increased.
The invention is devised in view of the above problems, and it is intended to provide a method of arranging a microsphere by means of liquid, microsphere arranging apparatus and semiconductor device such that the manufacturing cost in the bump electrode forming process can be reduced, the entire composition of the process can be simplified, and the conductive liquid and microspheres can be recycled without being wasted.